1. Field of the Invention
The present invention generally relates to a method and apparatus of alignment marks on a semiconductor wafer that allow the alignment marks to be used when the semiconductor wafer or alignment marks are rotated 180 degrees from its expected orientation.
2. Description of the Related Art
The problem of the prior art alignment marks used in photo lithography tools, (for example, with ASLM™), is that improperly oriented alignment marks do not work properly. For example, if alignment marks are inadvertently rotated 180 degrees from expected orientation, the data contained in the alignment marks cannot be read properly.
Some solutions to this problem have been to buy new masks with properly placed alignment marks or to drop in additional alignment marks of a second mask. Problems with these known solutions are that new masks can be very costly ($100K or more in some cases) and dropping alignment marks off a second reticle can degrade the overall overlay performance by increasing the number of overlay error sources.
The solution to this problem should address both of these deficiencies and provide a significant cost improvement rather than fixing and/or reordering new masks. The solution should provide that there is no degradation in overlay performance. The solution can not only address yield issues, but can also prevent devices from being defective.